18650 battery 3500mah lithium

Principle and process flow of 18650 battery 3500mah lithium

The principle, formula and process flow of 18650 battery 3500mah lithium, and detailed manufacturing parameters are fully explained

18650 battery 3500mah lithium are a type of secondary battery (rechargeable battery), which mainly relies on the intercalation and deintercalation of Li+ between two electrodes to work. With the continuous development of downstream industries such as power vehicles, the production scale of 18650 battery 3500mah lithium is expanding.

1. Working principle

1. Positive electrode structure

LiCoO2+conductive agent+adhesive (PVDF)+current collector (aluminum foil)

2. Negative electrode structure

Graphite+conductive agent+thickener (CMC)+binder (SBR)+current collector (copper foil)

3. Working principle

3.1 Charging process

A power source charges the battery. At this time, the electrons e on the positive electrode run from the external circuit to the negative electrode, and the positive lithium ions Li+ "jump" into the electrolyte from the positive electrode, "climb" through the tortuous holes on the separator, "swim" to the negative electrode, and combine with the electrons that have already run over. At this time:

The reaction on the positive electrode is:

3.2 Battery discharge process

Discharge has constant current discharge and constant resistance discharge. Constant current discharge is actually adding a variable resistor that can change with the voltage change in the external circuit. The essence of constant resistance discharge is to add a resistor to the positive and negative electrodes of the battery to allow electrons to pass through. It can be seen that as long as the electrons on the negative electrode cannot run from the negative electrode to the positive electrode, the battery will not discharge. Electrons and Li+ move together, in the same direction but different paths. During discharge, electrons run from the negative electrode to the positive electrode through the electronic conductor, and lithium ions Li+ "jump" from the negative electrode into the electrolyte, "climb" through the tortuous small holes on the separator, "swim" to the positive electrode, and combine with the electrons that have already run over.

3.3 Charge and discharge characteristics

The positive electrode of the battery cell uses LiCoO2, LiNiO2, and LiMn2O2. Among them, LiCoO2 is a crystal with a very stable layer structure, but when x Li ions are taken away from LiCoO2, its structure may change, but whether it changes depends on the size of x.

Through research, it is found that when x>0.5, the structure of Li1-xCoO2 is extremely unstable, and the crystal collapse will occur, which is manifested externally as the collapse of the battery cell. Therefore, the x value in Li1-xCoO2 should be controlled by limiting the charging voltage during the use of the battery cell. Generally, if the charging voltage is not greater than 4.2V, then x<0.5, and the crystal form of Li1-xCoO2 is still stable.

The negative electrode C6 itself has its own characteristics. After the first formation, the Li in the positive electrode LiCoO2 is charged into the negative electrode C6, and when discharged, Li returns to the positive electrode LiCoO2. However, after the formation, a part of Li must be left in the center of the negative electrode C6 to ensure the normal embedding of Li in the next charge and discharge, otherwise the collapse of the battery cell is very short. In order to ensure that a part of Li remains in the negative electrode C6, it is generally achieved by limiting the lower limit voltage of discharge: the upper limit voltage of safe charging is ≤4.2V, and the lower limit voltage of discharge is ≥2.5V.

The principle of the memory effect is crystallization, which almost never occurs in lithium batteries. However, the capacity of 18650 battery 3500mah lithium will still decrease after repeated charging and discharging, and the reasons are complicated and diverse. The first is the change of the positive and negative electrode materials themselves. From a molecular level, the hole structure that contains lithium ions on the positive and negative electrodes will gradually collapse and block; from a chemical point of view, it is the active passivation of the positive and negative electrode materials, and the side reactions will produce other stable compounds. Physically, there will also be the gradual peeling of the positive electrode materials, which will eventually reduce the number of lithium ions that can move freely in the battery during the charging and discharging process.

Overcharging and overdischarging will cause permanent damage to the positive and negative electrodes of 18650 battery 3500mah lithium. From a molecular level, it can be intuitively understood that overdischarging will cause the negative electrode carbon to release excessive lithium ions and cause its layer structure to collapse, and overcharging will force too many lithium ions into the negative electrode carbon structure, so that some of the lithium ions can no longer be released.

Unsuitable temperature will trigger other chemical reactions inside the lithium-ion battery to produce compounds that we do not want to see, so a protective temperature control barrier or electrolyte additive is set between the positive and negative electrodes of many 18650 battery 3500mah lithium. When the battery temperature rises to a certain level, the pores of the composite membrane close or the electrolyte denatures, the internal resistance of the battery increases until the circuit is broken, and the battery no longer heats up, ensuring that the battery charging temperature is normal.

2. Lithium battery formula and process flow

1. Positive and negative electrode formula

1.1 Positive electrode formula: LiCoO2+conductive agent+adhesive+current collector (aluminum foil)

LiCoO2(10μm):96.0%

Conductive agent (CarbonECP) 2.0%

Adhesive (PVDF761) 2.0%

NMP (adding adhesiveness): the weight ratio of solid matter is about 810:1496

a) The viscosity of the positive electrode is controlled at 6000cps (temperature 25 rotor 3);

b) The NMP weight must be properly adjusted to meet the viscosity requirements;

c) Pay special attention to the effects of temperature and humidity on viscosity

Positive electrode active material:

Lithium cobalt oxide: positive electrode active material, lithium ion source, and lithium source for battery improvement. Non-polar substance, irregular shape, particle size D50 is generally 6-8μm, water content ≤0.2%, generally alkaline, pH value is about 10-11.

Lithium manganate: non-polar substance, irregular shape, particle size D50 is generally 5-7μm, water content ≤0.2%, generally weakly alkaline, pH value is about 8.

Conductive agent: chain, water content <1%, particle size is generally 1-5μm. Generally, superconducting carbon black with excellent conductivity is used, such as Ketjen Carbon ECP and ECP600JD. Its function: improve the conductivity of positive electrode materials, compensate for the electronic conductivity of positive electrode active materials; improve the liquid absorption of electrolyte of positive electrode sheets, increase reaction interface, and reduce polarization.

PVDF adhesive: non-polar substance, chain, molecular weight ranges from 300,000 to 3,000,000; molecular weight decreases after water absorption, and viscosity becomes worse. Used to bond lithium cobalt oxide, conductive agent and aluminum foil or aluminum mesh together. Common brands include Kynar761.

NMP: a weakly polar liquid used to dissolve/swell PVDF and dilute the slurry.

Current collector (positive lead): made of aluminum foil or aluminum tape.

1.2 Negative electrode formula: graphite + conductive agent + thickener (CMC) + binder (SBR) + current collector (copper foil)

Negative electrode material (graphite): 94.5%

Conductive agent (CarbonECP): 1.0% (Ketjen superconducting carbon black)

Binder (SBR): 2.25% (SBR = styrene-butadiene rubber latex)

Thickener (CMC): 2.25% (CMC = sodium carboxymethyl cellulose)

The weight ratio of water: solid matter is 1600:1417.5

a) Negative electrode viscosity control 5000-6000cps (temperature 25 rotor 3)

b) The water content needs to be properly adjusted to meet the viscosity requirements;

c) Pay special attention to the effect of temperature and humidity on viscosity

2. Positive and negative mixing

Graphite: Negative active material, the main substance that constitutes the negative electrode reaction; it is mainly divided into two categories: natural graphite and artificial graphite. Non-polar substances are easily contaminated by non-polar substances and easily dispersed in non-polar substances; they are not easy to absorb water and are not easy to disperse in water. Contaminated graphite is easy to regroup after being dispersed in water. The general particle size D50 is about 20μm. The particle shapes are diverse and mostly irregular, mainly spherical, flaky, fibrous, etc.

Conductive agent: Its functions are:

a) Improve the conductivity of the negative electrode sheet and compensate for the electronic conductivity of the negative electrode active material.

b) Improve the reaction depth and utilization rate.

c) Prevent the formation of dendrites.

d) Utilize the liquid absorption capacity of conductive materials, improve the reaction interface, and reduce polarization. (You can choose to add or not according to the distribution of graphite particle size).

Additives: Reduce irreversible reactions, improve adhesion, improve slurry viscosity, and prevent slurry deposition.

Thickener/anti-deposition agent (CMC): polymer compound, easily soluble in water and polar solvents.

Isopropyl alcohol: a weak polar substance, which can reduce the polarity of the binder solution and improve the compatibility of graphite and the binder solution after addition; it has a strong defoaming effect; it is easy to catalyze the network crosslinking of the binder and improve the bonding strength.

Ethanol: a weak polar substance, which can reduce the polarity of the binder solution and improve the compatibility of graphite and the binder solution after addition; it has a strong defoaming effect; it is easy to catalyze the linear crosslinking of the binder and improve the bonding strength (the effects of isopropyl alcohol and ethanol are essentially the same. When mass production is carried out, the cost factor can be considered and then which one to add can be selected).

Water-based binder (SBR): Bond graphite, conductive agent, additives and copper foil or copper mesh together. Small molecule linear chain emulsion, very soluble in water and polar solvents.

Deionized water (or distilled water): diluent, added in moderation to change the fluidity of the slurry.

Negative lead: made of copper foil or nickel strip.

2.1 Positive electrode mixture:

2.1.1 Pretreatment of materials

1) Lithium cobalt oxide: dehydration. Generally, it is baked at 120°C and normal pressure for about 2 hours.

2) Conductive agent: dehydration. Generally, it is baked at 200°C and normal pressure for about 2 hours.

3) Binder: dehydration. Generally, it is baked at 120-140°C and normal pressure for about 2 hours. The baking temperature depends on the molecular weight.

4) NMP: dehydration. Use dry molecular sieve for dehydration or use special material collection facilities for direct use.

2.1.2 Material ball milling:

1) After 4 hours, separate the ball milling by sieving;

2) Pour LiCoO2 and CarbonECP into the material barrel, add grinding balls (dry material: grinding balls = 1:1), and ball mill on the roller bottle and the speed is controlled at more than 60rmp

2.1.3 Material blending:

1) Dissolution of the binder (according to the standard concentration) and heat treatment.

2) Ball milling of lithium cobalt oxide and conductive agent: Make the powder begin to mix, lithium cobalt oxide and conductive agent bond together, improve the aggregation effect and conductivity. After being prepared into slurry, it will not be dispersed in the binder alone. The ball milling time is generally about 2 hours. To prevent impurities from being mixed in, agate balls are generally used as ball milling mesons.

2.1.4 Dispersion and wetting of dry powder:

Principle: When solid powder is placed in the air, it will adsorb part of the air on the surface of the solid over time. After the liquid binder is added, the liquid and gas begin to compete for the solid surface; if the adsorption force between the solid and the gas is stronger than that between the solid and the liquid, the liquid cannot wet the solid; if the adsorption force between the solid and the liquid is stronger than that between the solid and the gas, the liquid can wet the solid and squeeze out the gas.

When the wetting angle is ≤90°, the solid is wetted. When the wetting angle is >90°, the solid is not wetted.

All members of the positive electrode material can be wetted by the binder solution, so the dispersion of the positive electrode powder is relatively simple.

The influence of dispersion method on dispersion:

1) Static method (long time, poor effect, but does not damage the original structure of the material);

2) Mixing method: self-rotation or self-rotation plus revolution (short time, good effect, but may damage the structure of some materials).

The influence of mixing paddle on dispersion speed: Mixing paddles generally include snake-shaped, butterfly-shaped, spherical, paddle-shaped, gear-shaped, etc. Generally, snake-shaped, butterfly-shaped, and paddle-shaped mixing paddles are used to deal with materials or ingredients with high dispersion difficulty in the initial stage; spherical and gear-shaped paddles are used for conditions with low dispersion difficulty and have good effects.

The influence of mixing speed on dispersion speed. Generally speaking, the higher the mixing speed, the faster the dispersion speed, but the greater the damage to the material's own structure and the equipment.

The influence of concentration on dispersion speed. Generally speaking, the lower the slurry concentration, the faster the dispersion speed, but too thin will lead to material waste and increased slurry deposition.

The influence of concentration on bonding strength. The higher the concentration, the greater the flexibility and bonding strength; the lower the concentration, the lower the bonding strength.

The effect of vacuum on dispersion speed. High vacuum is conducive to the discharge of gas from the gaps and surface of the material, reducing the difficulty of liquid adsorption; the difficulty of uniform dispersion of the material will be greatly reduced when the material is completely weightless or the gravity is reduced.

The effect of temperature on dispersion speed. At a suitable temperature, the slurry has good fluidity and is easy to disperse. Too hot slurry is easy to crust, and too cold slurry will greatly reduce its fluidity.

Dilution: Adjust the slurry to a suitable concentration for easy coating.

2.1.5 Operation process

a) Pour NMP into the power mixer (100L) to 80°C, weigh PVDF and add it, and start the machine; parameter setting: speed 25±2 rpm, stirring for 115-125 minutes;

b) Turn on the cooling system, add the ground positive electrode dry material in four equal times, each time interval 28-32 minutes, add NMP according to the material requirements for the third addition, and add NMP after the fourth addition; power mixer parameter setting: speed 20±2 rpm

c) 30±2 minutes after the fourth addition, high-speed stirring is carried out, the time is 480±10 minutes; power mixer parameter setting: revolution 30±2 rpm, autorotation 25±2 rpm;

d) Vacuum mixing: Connect the power mixer to vacuum, maintain the vacuum degree at -0.09Mpa, and mix for 30±2 minutes; Power mixer parameter settings: revolution 10±2 minutes, autorotation 8±2 rpm

e) Take 250-300 ml of slurry and use a viscometer to measure the viscosity; Test conditions: rotor number 5, speed 12 or 30rpm, temperature range 25°C;

f) Take the positive electrode material out of the power mixer for colloid milling and sieving, and label it on the stainless steel basin at the same time. After explaining to the operator of the slurry pulling equipment, it can flow into the slurry pulling process.

2.1.6 Precautions

a) After completion, clean up the machinery and equipment and the working environment;

b) When operating the machine, pay attention to safety to prevent head injuries.

2.2 Negative electrode mixing

2.2.1 Pretreatment of materials:

1) Graphite:

A. Mix to homogenize the materials and improve consistency.

B. Bake at 300~400°C under normal pressure to remove surface oily substances, improve compatibility with water-based adhesives, and round the edges and corners of graphite surface (some materials are not allowed to be baked to maintain surface characteristics, otherwise the performance will decrease).

2) Water-based adhesive: Proper dilution to improve dispersion.

2.2.2 Blending, wetting and dispersion:

1) Graphite and adhesive solution have different polarities and are not easy to disperse.

2) Graphite can be initially moistened with alcohol-water solution and then mixed with adhesive solution.

3) The mixing concentration should be appropriately reduced to improve dispersion.

4) The dispersion process is to reduce the distance between polar and non-polar substances and improve potential energy or surface energy, so it is considered an endothermic reaction, and the overall temperature decreases during mixing. If conditions permit, the mixing temperature should be appropriately increased to make heat absorption easier, improve mobility and reduce the difficulty of dispersion.

5) If the mixing process is combined with a vacuum degassing process to remove gas and promote solid-liquid adsorption, the effect will be better.

6) The dispersion principle and dispersion method are the same as those in the positive electrode matching

2.2.3 Dilution:

Adjust the slurry to a suitable concentration for easy coating.

2.2.4 Material ball milling

1) Pour the negative electrode and KetjenblackECP into

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